Sensor for non-invasive and continuous determination of the duration of arterial pulse waves

ABSTRACT

A sensor for non-invasive and continuous determination of the duration of arterial pulse waves is provided in which at least two spaced apart piezoelectric pressure sensors are disposed in succession in the flow direction of the artery, with the sensor being provided with pressure sensitive surfaces and being integrated in a casing. The piezoelectric pressure sensors are provided with a pressure-sensitive, strip-shaped surface, with the strips each being disposed in their longitudinal extension perpendicular to the flow direction of the artery. The casing is provided with at least two recesses adapted to the contours of the strip-shaped surfaces into which the pressure-sensitive, strip-shaped surfaces of the pressure sensors are disposed flush to the surface of the casing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sensor for non-invasive andcontinuous determination of the duration of arterial pulse waves.

2. Description of the Related Art

A desirable goal for many manufacturers of blood-pressure measuringdevices is continuous non-invasive measurement of human blood pressurewithout the aid of uncomfortable compression cuffs.

It has been known for quite some time that human blood pressure,differing from individual to individual, correlates to the velocity ofpulse waves. Hitherto this fact could not be exploited for continuousmeasurement of blood pressure, because there are no reliable,inexpensive sensors available for determination of the pulse-wavevelocity.

Hitherto, attempts have been made to determine the pulse-wave velocity,which is about 10 m/s at the wrist, via the change in the color, theform or in the electric resistance of the skin. Attempts have also beenmade to measure the pulse-wave velocity with the aid of the ultrasonicDoppler method, a reliable but not exactly inexpensive method. One suchattempt is described in DE-OS-1 905 620. Two spaced apart piezoelectricoscillator systems, the conical-shaped sound of which irradiates ato-be-examined vessel and a Doppler reception device permitdetermination of the vessel-wall velocity with which the vessel wall isextended by the blood-pressure waves flowing through the vessel.Ultimately information on the pulse-wave velocity is obtained via aspecial evaluation algorithm.

Another example for determining the pulse-wave velocity is indicated inU.S. Pat. No. 4,245,648 describing a process and a device for measuringblood pressure and for determining the pulse rate. Twopressure-sensitive sensors housed in an arm cuff are placed along ablood-conducting vessel. The increase-in-pressure values determined inintervals can be utilized for calculating the pressure-wave velocity.However, disadvantageous is the large size of the device making itimpossible to use at sites that are difficult to reach. Furthermore,application of the device involves considerable motoric impediment.

DESCRIPTION OF THE INVENTION

The object of the present invention is to further improve the sensorknown from U.S. Pat. No. 4,245,648 in such a manner that it is designedso small and compact that it can be combined, by way of illustration,with a wristwatch.

The solution to the object of the present invention is set forth inclaim 1. Advantageous embodiments are the subject matter of thesubclaims.

An element of the present invention is that a sensor for non-invasiveand continuous determination of the duration of arterial pulse waves, inwhich at least two piezoelectric pressure sensors are placed atintervals in succession in the running direction of the arteries, withthe sensor being provided with pressure sensitive surfaces andintegrated in a casing, is designed in such a manner that thepiezoelectric pressure sensors have a pressure-sensitive, strip-shapedsurface which are each placed in their longitudinal extensionperpendicular to the running direction of the arteries, and that thecasing is provided with at least two recesses adapted to the contours ofthe strip-shaped surfaces into which the pressure-sensitive,strip-shaped surfaces of the pressure sensors are disposed flush to thecasing surface.

The invented sensor determines non-invasively the pressure pulsations ofthe arteria radialis preferably at the wrist level at two closelyadjacent positions of which the one is located more proximally and theother more distally at the measuring point of the arteria radialis inthe wrist region. The pulse-wave velocity and therefore, usingpreviously patient related calibration, the average blood pressure canbe determined from the time lag of the pulse maximum of the twomeasuring positions. The level of the systolic and diastolic pressurecan be immediately determined from the measured difference between thepulse-pressure maximum and pulse-pressure minimum at one of thepositions. The device is so small that it can be constantly worn on thewrist like a wristwatch, thereby largely avoiding any discomfort to thepatient.

The sensor immediately determines the difference in the duration of thepulse wave propagating in the artery thereby setting strictermeasurement criteria so that the invented sensor works more accuratelythan hitherto known measurement methods in which the indirectdetermination of the pulse wave occurs via the color, resistance, formof the skin. Inexpensive mass production of the sensor is also feasible.

For determination of the pressure, the invented sensor is provided withat least two separately operating pressure sensors which each areprovided with a pressure-sensitive, strip-shaped surface and which aredisposed in their longitudinal extension perpendicular to the runningdirection of the artery. The individual pressure sensors composed ofpiezoelectric material are integrated in a semi-cylindrical casing insuch a manner that they are inserted in the angular running recesses inthe casing wall at the convex, hemispherical nappe of the semi-cylinder.

Therefore, the individual pressure-sensitive surfaces essentially followthe convex-shaped hemispherical surface contour of the semi-cylinderwhich is pressed against the surface of the skin in such a manner thatthe curved pressure-sensitive surfaces intersect perpendicularly therunning direction of the artery with radial polarization.

The purpose of the convex curvature of the semi-cylinder and thepressure sensitive surfaces connected thereto is to ensure, upon lightlypressing the sensor casing against the natural form of the surface ofthe skin, improved adaptation to the measuring site and therewithimproved mechanical contacting of the measuring object. Furthermore, thepurpose of the semi-cylindrical surface contour of the sensor casing islargely insensitive to overturning at the longitudinal axis.

Moreover, the pressure-sensitive surfaces of the pressure sensor has tobe designed narrow in the direction of the running direction of theartery so that a small as possible duration of the pulse wave at eachindividual sensor area can be attained, thereby permitting obtaininghigh temporal resolution. The distance between the two pressuresensitive surface areas has to be selected so small that both pressuresensors still lie close to the surface in the region of the course ofthe arteria radialis. Only in this manner, can both pressure sensorsdetect the same temporal pulse duration. On the other hand, the distancehas to be large enough in order to be able to still pick up the time lagof the pulse maximum between the two pressure sensors. Tests have shownthat these conditions can be realized if the individualpressure-sensitive surfaces have a width of 1 mm and are spaced 1 cmapart.

The individual piezoelectric pressure sensors are composed ofpiezoelectric material and their surface facing the artery projectsthrough the aforementioned recesses worked into the convex shaped casingwall. In this way. the individual piezo electric pressure sensors assumea hemispherical shape, in which upon external pressure polarizationcharges, which lead to an electric voltage between the pressure sensorsurfaces, are released between their external and their internal surfaceproportional to the application of mechanical pressure or tension.

In view of the fact that when pressure is applied, a high voltage isgenerated between the surfaces of the piezoelectric pressure sensors byrelatively small charges, low-frequency pressure fluctuations, such asarterial pressure pulsations, can no longer be detected if the receiveris directly connected to a low-ohmic signal processing system. Thereforea preamplifier with a high as possible input resistor and a low aspossible output resistor is required which is placed as close aspossible to the piezoelectric element.

Preferably, a simple, as small as possible to-be-realized impedanceconverter circuit, which by way of illustration is composed of a fieldeffect transistor and two resistors, which are immediately integratedinside the sensor casing, should be provided for each piezoelectricpressure sensor. Due to its high-ohmic input, the entire sensorelectronics should be completely screened off against electricinterferences.

BRIEF DESCRIPTION OF THE DRAWING

The present invention is made more apparent in the following usingpreferred embodiments with reference to the drawings by way of example.Depicted is in:

FIG. 1 an external view of an invented sensor and

FIG. 2 a longitudinal section of an invented sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an invented preferred embodiment of a sensor fornon-invasive and continuous determination of the duration of arterialpulse waves, which is provided with a semi-cylindrical casing 1. Twospaced apart piezoelectric pressure sensors 2 are worked into thehemispherical, convex outer contour of the casing. The invented sensoris pressed with its convex, hemispherical semi-cylindrical surface atthe area of an artery 3 through which pulse waves 4 travel. The sensorcasing 1 is preferably provided with a radius of curvature of 2.5 mm inthe convex, hemispherical nappe area and of about 14 mm on the overalllength of the casing. Preferably two angularly running recesses, throughwhich piezoelectric pressure sensor materials 2 project through from theinside, are provided in the convex, hemispherical nappe area of casing1.

FIG. 2 shows a longitudinal representation through an invented sensor,whose convex hemispherical nappe surface faces artery 3. As therepresentation shows. The top side of the semi-cylindrical casing 1 isinterrupted by two recesses through which a piezoelectric sheeting 5projects from the inside. The piezopolymer sheeting 5 slightly protrudesbeyond the surface of casing 1. The outward facing pressure-sensitivesurface of the sheeting is, in addition, metallized and thus inelectrical contact with the metal sensor housing. Preferably thecontacting occurs via a press contact or adhesion contacting.

In order to tap polarization charges generated on the bottom side of thesheeting due to the deformation of the piezopolymer sheeting,hemispheric disks of a conductive elastomer 6 are provided, whichconnect the piezopolymer sheeting to an impedance converter circuitwhich is placed on a substrate 7 for each individual pressure sensor.The voltage tapping at the internal side of each pressure sensor occursin the area of the recesses via the respective conducting elastomerhemispheric disk 6 of approximately 1 mm thickness. The elastomerhemispheric disks 6 are contacted by means of press contacting to thenon-metallized internal side of the piezo sheeting 5 with the respectivesignal input of both integrated impedance converter circuits on thesubstrate 7. Simultaneously, due to the elastic lining of the rear sideof the sheeting by using elastic elastomer hemispheric disks ensuresthat the sheeting surface is pressed slightly outward at the recessesand therefore is essentially flush on the front to the surface of thecasing or slightly raised.

Evaluation of the time lag of the pulse maxima occurs by means ofdifferentiating the impedance converter/output signals. The time-laggedzero passes of the two differentiated pulse pressure courses can beutilized to start or stop an electronic stopwatch. The small size of thesensor could permit wearing the sensor on a wristband together with aminiaturized evaluation electronics including a display for showing thepulse frequency and blood pressure.

What is claimed is:
 1. A sensor for non-invasive and continuousdetermination of a duration of arterial pulse waves of an artery, thesensor comprising: at least two spaced apart piezoelectric pressuresensors, each of which is provided with a pressure-sensitive,strip-shaped surface; a casing having at least two recesses adapted tocontours of said strip-shaped surfaces, said pressure-sensitive,strip-shaped surfaces being integrated in said casing within saidrecesses so as to be disposed flush with an outer surface of saidcasing; and wherein said at least two spaced apart piezoelectricpressure sensors are disposed in succession in said casing such thatsaid respective pressure sensors are arrangeable to extendlongitudinally in a direction perpendicular to a flow direction of theartery.
 2. The sensor according to claim 1, wherein said casing ismetallic.
 3. The sensor according to claim 1, wherein saidpressure-sensitive, strip-shaped surfaces of said piezoelectric pressuresensors have a strip width which is smaller than a spacing between saidpiezoelectric pressure sensors.
 4. The sensor according to claim 2,wherein said pressure-sensitive, strip-shaped surfaces of saidpiezoelectric pressure sensors have a strip width which is smaller thana spacing between said piezoelectric pressure sensors.
 5. The sensoraccording to claim 3, wherein said space between said at least twopiezoelectric pressure sensors is about 1 cm and said strip width isabout 1 mm.
 6. The sensor according to claim 4, wherein said spacebetween said at least two piezoelectric pressure sensors is about 1 cmand said strip width is about 1 mm.
 7. The sensor according to claim 2,wherein said casing has a semi-cylindrical shape, said recesses beingprovided in a convex hemispherical nappe of said casing such that saidrecesses run angularly at least partly about a cylindrical axis of saidcasing.
 8. The sensor according to claim 7, wherein a radius ofcurvature of said semi-cylindrical shape is about 2.5 mm.
 9. The sensoraccording to claim 1, wherein at least one of said two piezoelectricpressure sensors includes a piezopolymer sheeting in an area of saidpressure-sensitive, strip-shaped surface.
 10. The sensor according toclaim 9, wherein said piezopolymer sheeting is a polyvinylidenefluoridehaving a metal coating at least on a side facing said casing in saidarea of said pressure-sensitive, strip-shaped surface.
 11. The sensoraccording to claim 1, further comprising impedance converter circuitsintegrated within said casing and coupled to respective ones of said atleast two piezoelectric pressure sensors for preamplifying piezoelectricsignals from said pressure sensors.
 12. The sensor according to claim11, further comprising conductive elastic elements which respectivelyprovide an electrical contact between an inside pressure sensitivesurface of each of said piezoelectric pressure sensors and an associatedone of said impedance converter circuits.
 13. The sensor according toclaim 12, wherein said conductive elastic elements are composed ofconductive silicon rubber.
 14. The sensor according to claim 1, furthercomprising one of a wristband and wristwatch in which said sensor isarranged, said one of said wristband and wristwatch being configured toallow for pulse frequency and blood pressure measurement of the arterywhen used.
 15. The sensor according to claim 1, wherein in order todetermine an interval of a pulse wave maxima in an arteria radialis ofthe artery, an interval between zero crossings of a first temporalderivation of sensor signals from said two spaced apart piezoelectricpressure sensors is registered.
 16. The sensor according to claim 1,wherein said casing has a semi-cylindrical shape.